The present invention relates to time base correction for video tape recorders (VTR), and more particularly to a heterodyne color signal recognition circuit to provide automatic time base correction mode selection in a video processor.
A video tape recorder (VTR) is an electro/mechanical system in which a quality video signal from a camera or off a remote feed is recorded on an elastic ribbon of tape which is moved back and forth between spinning components. Due to the tolerances of the tape and mechanical components the video signal from the VTR on playback has time base instability. In many broadcast VTRs the video signal is recorded directly onto the tape, maintaining the phase relationship between the color subcarrier and the horizontal sync (455/2). However less expensive VTRs separate the luminance and chrominance components. The chrominance components are stripped by a filter and heterodyned to a lower frequency for recording. This "color-under", or "heterodyne", technique effectively eliminates the phase relationship between the color subcarrier and horizontal sync.
A time base corrector (TBC) is placed between the VTR and the rest of a video system to remove the time base errors induced into the recorded video signal by the stretching and slipping of the tape as it moves past the record/playback heads and by the tolerances of the spinning components, particularly the record/playback head scanner. Some VTRs include the TBC as an integral part of the VTR system. However in other VTR systems the TBC is separate and may even be shared among a number of VTRs and other video sources. Since the characteristics of the various video sources may be different, the TBC must be switched between different modes of processing. This mode switching has conventionally been done manually depending upon whether the input signal is a stable signal from a direct feed source, a recorded signal from a direct color VTR source, or a heterodyned signal from a color-under VTR source. Use of incorrect processing may result in an improper output video signal or degraded video signal quality.
A first attempt at automatic recognition of a heterodyne VTR video source was to form an electronic window from an oscillator signal locked to the video signal subcarrier. The electronic window is established about where the leading edge of the horizontal sync pulse of the composite sync is predicted to be. For each sync pulse which is outside the window an up/down counter is incremented depending upon whether the sync pulse precedes or succeeds the window. For low frequency time base errors generally all the sync pulses which fall outside the window are at one side, i.e., are cumulative. The absolute value from the up/down counter over a selected number of video lines is compared with a threshold value. When the threshold value is exceeded in succeeding video fields, a non-direct color indicator is generated to switch in the TBC circuit for the video signal. However, especially with the new VTRs which eliminate the low frequency problem, the sync pulses may occur on both sides of the window throughout the video field due to high frequency time base errors, resulting in an absolute value from the up/down counter which does not exceed the threshold value. Thus the non-direct color source, i.e., heterodyned VTR, is not detected. Then the operator is relegated to manually switching in the TBC whenever a heterodyned VTR is used as a video source.
What is desired is a heterodyne color signal recognition circuit which automatically detects the presence of a non-direct, heterodyned color source under all conditions.